fix User cannot be blank or null

[Absh-Day]

https://issues.jenkins.io/browse/JENKINS-70276
When using token auth without user/pass, we used to get error 'User cannot be blank or null' in job configuration as well as builds. this PR fixes that.

Testing done

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[Absh-Day]

@ltamaster could you merge this PR? This is an urgent fix.

[funkboy3]

Any chance someone could merge this so the fix can get out into the wild?